This invention was supported in part with funding provided by the National Science Foundation; the government has certain rights in the invention.
The formation of diaryl ethers from arenols and aryl groups bearing a leaving group generally requires activated substrates. For aryloxide nucleophiles, the reaction is promoted by copper salts. For instance, Ullman (Ber. 37:853) observed in 1904 that the presence of metallic copper greatly facilitates the substitution of a halogen atom on an aromatic ring with a phenolic oxygen. This method for the synthesis of diaryl ethers, referred to in the contemporary art as the xe2x80x9cUllmann reactionxe2x80x9d or xe2x80x9cUllmann condensationxe2x80x9d, has become widely used in both academic and industrial chemistry. For a review, see Moroz et al. (1974) Russ. Chem. Rev. 43:679.
The Ullmann reaction is typically carried out by heating the reactants at elevated temperatures, e.g., 150-300xc2x0 C., in the presence of a copper salt, which in some cases is present in only a catalytic amount; Ullmann reactions are usually conducted in a solvent, however, in certain cases a solvent is not utilized (the optimal temperature for Ullmann condensations conducted without solvent is typically in range of 180-220xc2x0 C.). To minimize oxidation of the phenolic reactant, the reactions are often performed under an inert atmosphere. See, for example, Weingarten et al. (1964) J. Org. Chem. 29:3624; and Moroz et al. supra.
The Ullmann reaction is widely used in industry, in particular, for the synthesis of various substituted diaryl ethers that are useful as pharmaceuticals, herbicides or insecticides, or as intermediates in the synthesis thereof. A wide variety of diaryl ethers have found use in, for instance, the synthesis of complex natural products. The reaction is also widely used in the general chemical industry. To illustrate, the Ullmann reaction serves as part of a method for the synthesis of phenyl ether (used to prepare heat exchangers and in the perfume industry), substituted phenyl ethers (monomers for thermostable polymers), and polyphenyl ethers, which have found important applications in, for example, oils for creation of ultrahigh vacuums, high-temperature greases, hydraulic liquids, etc.
The conditions used for the copper-mediated coupling of aryl halides and phenols according to the Ullmann reaction are usually harsh, requiring high temperatures and high-boiling, polar solvents (pyridine, collidine, DMF). The classical use of the reaction also has the usual requirement for stoichiometric (or greater) quantities of the copper reagent. The yields of the reaction are substrate dependent and are usually low for transformations involving highly functionalized substrates. For example, during the course of the total synthesis of such antibiotics as vancomycin, attempted Ullmann condensations for the coupling of advanced intermediates were unsuccessful (see, e.g., Williams (1984) Acc Chem. Res. 17:364). Indeed, in the synthesis of pharmaceutical agents, many of the desirable substituents on the substrate aryl moieties may not be sufficiently stable, even when protected, for use in the Ullmann reaction of the prior art. For instance, esters and anhydrides are not stable under the classical Ullmann reaction conditions.
Likewise, the conditions of the classical Ullmann reaction may be too harsh for use in a combinatorial approach to the synthesis of libraries of diaryl ethers and the like, particularly where labile groups are employed, for example, as linker groups or encoding tags (see: Ohlmeyer et al. (1993) PNAS 90:10922; and Brenner et al. (1992) PNAS 89:5381-5383).
Another problem with the classical Ullmann reaction is that many of the solvents that are characteristically relied upon are extremely hazardous. Disposal of reaction by-products, e.g., spent solvent, may accordingly pose significant obstacles, in terms of environmental safety and/or ultimate product cost, to reliance on the classical Ullmann reaction.
Recent efforts to develop Ullmann-type procedures which are applicable to more complex synthetic intermediates have met with only limited success (Evans et al. (1989) J. Am. Chem. Soc. 111:1063; and Boger et al. (1991) J. Org. Chem. 56:4204) or require the presence of an activating group on the substrate bearing the leaving group (for selected examples of activating groups, see Nicolaou et al. (1997) J. Am. Chem. Soc. 119:3421 and Rozanel""skaya et al. (1961) Zhur. Obschc. Khim. 31:758).
One aspect of the present invention relates to novel reaction conditions that allow the efficient synthesis of diaryl ethers from arenes bearing a leaving group and arenols under relatively mild conditions. Another aspect of the present invention relates to the discovery of the dramatic effects of aryl carboxylic acid additives on copper-catalyzed Ullmann-type couplings.
One aspect of the present invention provides a cross-coupling reaction, represented in the general formula: 
wherein
Ar and Arxe2x80x2 independently represent optionally substituted aryl or heteroaryl groups;
YH represents a substituent of Arxe2x80x2 that includes a nucleophilic group, or a group that can be rendered nucleophilic;
X represents a leaving group which can be substituted by the nucleophilic group of Y in a transition metal-catalyzed reaction;
the transition metal catalyst is a complex which catalyzes formation of ArYArxe2x80x2 from ArX and Arxe2x80x2YH;
the metal salt has an anionic portion that is sufficiently basic to neutralize the HX produced in the reaction and/or deprotonate Arxe2x80x2YH and thereby render it a better nucleophile, and the metal salt comprises a soft cation selected from the alkali metal or alkaline earth series, e.g., Rb, Cs, Fr, Sr, Ba or Ra.
In another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, is included in the reaction mixture.
In another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, wherein a stoichiometric amount of a carboxylic acid, e.g., an aryl carboxylic acid, is included in the reaction mixture.
In yet another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, wherein a catalytic amount, e.g., 5 mol %, of a Lewis basic additive, e.g., an ester, and a stoichiometric amount of a carboxylic acid, e.g., an aryl carboxylic acid, are included in the reaction mixture.
In another embodiment of the present invention the transformation depicted above, in any of the aforementioned embodiments, occurs in an aprotic, non-polar solvent, e.g., toluene, at about 110 C.
The ability to provide an Ullmann synthesis method that can be carried out under mild conditions and/or with non-polar, aprotic solvents has broad application, especially in the agricultural, polymer, and pharmaceutical industries. The present invention provides such an Ullmann synthesis method.
One aspect of the present invention provides a mild and general procedure for the formation of diaryl ethers and the like. The subject procedure, set out in more detail below and in the Examples, is characterized by one or more of the following features:
(i) the reaction uses a sub-stoichimetric amount of a metal catalyst, such as a copper catalyst;
(ii) the reaction uses an alkali salt, the cationic portion being a soft cation of the alkali metal or alkaline earth series and (preferably) having electrons in the 4th or higher quantum level, and the anionic portion being basic enough to deprotonate a phenolic group or the like (e.g., the nucleophilic group of Arxe2x80x2YH below), wherein the alkali salt eliminates the need to form a phenoxide anion or the like prior to reaction;
(iii) the reaction can be run with good yield in a non-polar, aprotic solvent; and
(iv) the reaction can be run with good yield at reaction temperatures below 130 C.
Based on the present experimental observations, the subject method may be generally represented by the following reaction scheme: 
wherein
Ar and Arxe2x80x2 independently represent optionally substituted aryl or heteroaryl groups;
YH represents a substituent of Arxe2x80x2 that includes a nucleophilic group, or a group that can be rendered nucleophilic;
X represents a leaving group which can be substituted by the nucleophilic group of Y in a transition metal-catalyzed reaction;
the transition metal catalyst is a complex which catalyzes formation of ArYArxe2x80x2 from ArX and Arxe2x80x2YH;
the metal salt has an anionic portion that is sufficiently basic to neutralize the HX produced in the reaction and/or deprotonate Arxe2x80x2YH and thereby render it a better nucleophile, and the metal salt comprises a soft cation selected from the alkali metal or alkaline earth series, e.g., Rb, Cs, Fr, Sr, Ba or Ra.
In another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, is included in the reaction mixture.
In another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, wherein a stoichiometric amount of a carboxylic acid, e.g., an aryl carboxylic acid, is included in the reaction mixture.
In yet another embodiment of the present invention, the subject method is represented by the transformation above and the attendant definitions, wherein a catalytic amount, e.g., 5 mol %, of a Lewis basic additive, e.g., an ester, and a stoichiometric amount of a carboxylic acid, e.g., an aryl carboxylic acid, are included in the reaction mixture.
In another embodiment of the present invention the transformation depicted above, in any of the aforementioned embodiments, occurs in an aprotic, non-polar solvent, e.g., toluene, at about 110 C.
In certain embodiments, the present invention is represented by the generalized method depicted in scheme 1: 
wherein
Ar and Arxe2x80x2 independently represent optionally substituted monocyclic or polycyclic aromatic or heteroaromatic moieties;
X represents Cl, Br, I, xe2x80x94O3S(alkyl), or xe2x80x94O3S(aryl);
Y represents O, S, Se, NR, PR, or AsR;
Z represents H, a group that is lost under the reaction conditions to generate a negative charge on Y, or a group that is replaced by H under the reaction conditions;
R represents H, alkyl, aryl, aralkyl, heteroalkyl, heteroaryl, trialkylsilyl, acyl, or sulfonyl;
catalyst represents a transition metal complex;
co-catalyst represents a uncharged molecule comprising a Lewis basic pair of electrons; and
alkali or alkaline earth salt represents a salt selected from the group consisting of alkali and alkaline earth bicarbonates, carbonates, carboxylates, phosphates, alkoxides, silicates, amides, and sulfides.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein X represents Br or I.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Y represents O, S, Se, or NR.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Y represents O or S.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Y represents O.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Z represents H.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Ar and Arxe2x80x2 independently represent optionally substituted monocyclic aryl or heteroaryl moieties.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein Ar and Arxe2x80x2 independently represent optionally substituted phenyl.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs in a non-polar, aprotic solvent.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs in an aromatic hydrocarbon solvent.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs in toluene.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 75-150 C.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 90-135 C.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 100-120 C.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the alkali or alkaline earth salt comprises cesium.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the alkali or alkaline earth salt is cesium carbonate.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the co-catalyst is an ester.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the co-catalyst is an acetate.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the co-catalyst is ethyl acetate.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst comprises copper.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst is a copper salt.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst is selected from the group consisting of copper carboxylates, copper carbonates, copper sulfates, copper sulfonates, copper halides, copper trifluoroborates, and copper phosphates.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst is copper triflate.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 0.01 to 20 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 0.1 to 10 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 1 to 5 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein X represents Br or I; Y represents O, S, Se, or NR; Ar and Arxe2x80x2 independently represent optionally substituted monocyclic aryl or heteroaryl moieties; the reaction occurs in a non-polar, aprotic solvent; the reaction occurs at a temperature in the range of about 75-150 C; the alkali or alkaline earth salt comprises cesium; the co-catalyst is an ester; the catalyst comprises copper; and the catalyst and co-catalyst are independently present in the range of about 0.01 to 20 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein X represents Br or I; Y represents O or S; the reaction occurs in an aromatic hydrocarbon solvent; the alkali or alkaline earth salt is cesium carbonate; the reaction occurs at a temperature in the range of about 90-135 C; the co-catalyst is an acetate; the catalyst is selected from the group consisting of copper carboxylates, copper carbonates, copper sulfates, copper sulfonates, copper halides, copper trifluoroborates, and copper phosphates; and the catalyst and co-catalyst are independently present in the range of about 0.1 to 10 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, wherein X represents Br or I; Y represents O; Z represents H; the reaction occurs in toluene; the alkali or alkaline earth salt is cesium carbonate; the catalyst is copper triflate; the co-catalyst is ethyl acetate; the reaction occurs at a temperature in the range of about 100-120 C; and the catalyst and co-catalyst are independently present in the range of about 1 to 5 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 1 and the associated definitions, or any of the embodiments described above, wherein the reaction is intramolecular.
In certain embodiments, the present invention is represented by the generalized method depicted in scheme 2: 
wherein
Ar and Arxe2x80x2 independently represent optionally substituted monocyclic or polycyclic aromatic or heteroaromatic moieties;
X represents Cl, Br, I, xe2x80x94O3S(alkyl), or xe2x80x94O3S(aryl);
Y represents O, S, Se, NR, PR, or AsR;
Z represents H, a group that is lost under the reaction conditions to generate a negative charge on Y, or a group that is replaced by H under the reaction conditions;
R represents H, alkyl, aryl, aralkyl, heteroalkyl, heteroaryl, trialkylsilyl, acyl, or sulfonyl;
catalyst represents a transition metal complex;
co-catalyst represents a uncharged molecule comprising a Lewis basic pair of electrons;
acid activator is selected from the group consisting of carboxylic acids, amides, hydroxamic acids, phosphoric acids, phosphonic acids, phosphinic acids, sulfonic acids, sulfinic acids, sulfenic acids, alkylboronic acids, arylboronic acids, silicic acids, alcohols, and thiols; and
alkali or alkaline earth salt represents a salt selected from the group consisting of alkali and alkaline earth bicarbonates, carbonates, carboxylates, phosphates, alkoxides, silicates, amides, and sulfides.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein X represents Br or I.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Y represents O, S, Se, or NR.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Y represents O or S.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Y represents O.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Z represents H.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Ar and Arxe2x80x2 independently represent optionally substituted monocyclic aryl or heteroaryl moieties.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein Ar and Arxe2x80x2 independently represent optionally substituted phenyl.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs in a non-polar, aprotic solvent.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs in an aromatic hydrocarbon solvent.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs in toluene.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 75-150 C.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 90-135 C.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the reaction occurs at a temperature in the range of about 100-120 C.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the alkali or alkaline earth salt comprises cesium.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the alkali or alkaline earth salt is cesium carbonate.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the co-catalyst is an ester.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the co-catalyst is an acetate.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the co-catalyst is ethyl acetate.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the acid activator is selected from the group consisting of alkyl and aryl carboxylic acids, alkyl and aryl phosphonic acids, alkyl and aryl sulfonic acids, and alkyl and aryl boronic acids.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the acid activator is selected from the group consisting of alkyl and aryl carboxylic acids.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the acid activator is selected from the group consisting of aryl carboxylic acids.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the acid activator is 1-naphthoic acid.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst comprises copper.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst is a copper salt.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst is selected from the group consisting of copper carboxylates, copper carbonates, copper sulfates, copper sulfonates, copper halides, copper trifluoroborates, and copper phosphates.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst is copper triflate.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 0.01 to 20 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 0.1 to 10 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein the catalyst and co-catalyst are independently present in the range of about 1 to 5 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein X represents Br or I; Y represents O, S, Se, or NR; Ar and Arxe2x80x2 independently represent optionally substituted monocyclic aryl or heteroaryl moieties; the reaction occurs in a non-polar, aprotic solvent; the reaction occurs at a temperature in the range of about 75-150 C; the alkali or alkaline earth salt comprises cesium; the acid activator is selected from the group consisting of alkyl and aryl carboxylic acids, alkyl and aryl phosphonic acids, alkyl and aryl sulfonic acids, and alkyl and aryl boronic acids; the co-catalyst is an ester; the catalyst comprises copper; and the catalyst and co-catalyst are independently present in the range of about 0.01 to 20 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein X represents Br or I; Y represents O or S; the reaction occurs in an aromatic hydrocarbon solvent; the alkali or alkaline earth salt is cesium carbonate; the reaction occurs at a temperature in the range of about 90-135 C; the acid activator is selected from the group consisting of alkyl and aryl carboxylic acids; the co-catalyst is an acetate; the catalyst is selected from the group consisting of copper carboxylates, copper carbonates, copper sulfates, copper sulfonates, copper halides, copper trifluoroborates, and copper phosphates; and the catalyst and co-catalyst are independently present in the range of about 0.1 to 10 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, wherein X represents Br or I; Y represents O; Z represents H; the reaction occurs in toluene; the alkali or alkaline earth salt is cesium carbonate; the catalyst is copper triflate; the acid activator is 1-naphthoic acid; the co-catalyst is ethyl acetate; the reaction occurs at a temperature in the range of about 100-120 C; and the catalyst and co-catalyst are independently present in the range of about 1 to 5 mol % relative to Arxe2x80x2YZ.
In certain embodiments, the subject method is represented by scheme 2 and the associated definitions, or any of the embodiments described above, wherein the reaction is intramolecular.
As described in the Examples, during a study directed to finding new methods for the preparation of diaryl ethers, it was discovered that the reaction of a phenol with an aryl halide in DMF, using a catalytic amount of copper salt and cesium carbonate at 110xc2x0 C., gave the desired diaryl ether product in high yield. These reaction conditions, e.g., using cesium carbonate or its equivalent, proved to be unexpectedly efficient for this reaction with an unactivated aryl halide substrate.
The reaction using the alkali salt cesium carbonate was also carried out in less polar solvents such as THF, ethyl acetate, xylene and toluene in order to avoid the problems associated with the use of toxic, high-boiling or water soluble solvents such as DMF and pyridine. As described in the Examples, the modified Ullmann reaction was found to be effective for coupling a wide range of substituted aryl groups, activated and unactivated, and using a wide range of solvents not previously utilized with any efficiency in the Ullmann reaction conditions known in the art.
Another aspect of the invention relates to the discovery that the use of a reaction additive, namely a carboxylic acid or the like (an xe2x80x9cacid activatorxe2x80x9d), can improve the yield of an Ullmann reaction for less soluble and/or less nucleophilic embodiments of Arxe2x80x2YH, particularly in non-polar solvents. As described in the Examples, stoichiometric amounts of a carboxylic acid added to the reaction mixture can dramatically increase the conversion and yield for the coupling of less reactive phenols. In this regard, the activator is an acid selected to help solubilize key intermediates in the condensation reaction. In general, the activator will include a strongly acidic group, e.g., with a pKa less than 5, which can produce a conjugate base that, under the reaction conditions, is a strong enough Lewis base to donate an electron pair to the metal catalyst to form a coordinate bond with a cationic form of the metal. Moreover, the activator renders its complex with copper-phenoxide more soluble in the reaction solvent than the copper catalyst by itself. By use of such reaction additives, the resulting new procedure allows, for the first time, an efficient Ullmann reaction in non-polar, aprotic solvents and with less reactive and/or soluble phenols.
While not wishing to be bound by any particular theory, the nature of the cation of the alkali salt may play an important role during the formation or the solubilization of such an intermediate. Cesium phenoxides and carboxylates are known to be relatively soluble in organic solvents. See, for example, Hennings et al. (1997) J. Org. Chem. 62:2; Kwizinga et al. (1979) J. Chem. Soc., Chem. Commun, 286; and Zaugg, H. E., (1976) J. Org. Chem. 41:3419. In the subject method, the use of cesium carbonate (and the like) may enhance the solubility of the phenoxides, and of the possible intermediate 1, compared to the use of its potassium and sodium counterparts.
Moreover, the results presented in the Examples suggest that the role of an acid additive, e.g., 1-naphthoic acid, is not simply an acid-base effect only, but rather that it plays an integral role in the reaction. For example, addition of cesium naphthoate rendered reactive a previously unreactive mixture containing cesium phenoxide, 1-bromo-4-tert-butylbenzene, and 2.5 mol % (CuOTf)2.PhH. In addition, the use of this additive allowed the reaction to proceed using potassium carbonate as a base.
Definitions
For convenience, before further description of the present invention, certain terms employed in the specification, examples, and appended claims are collected here.
The term xe2x80x9csubstrate aryl groupxe2x80x9d refers to an aryl group containing an electrophilic atom which is susceptible to the subject cross-coupling reaction, e.g., the electrophilic atom bears a leaving group.
The terms xe2x80x9creactive aryl groupxe2x80x9d and xe2x80x9cnucleophilic aryl groupxe2x80x9d refer to an aryl group substituted with a nucleophilic moiety which can attack the electrophilic atom of the substrate aryl group and displace the leaving group in the subject cross-coupling reaction.
The substrate aryl group and nucleophilic aryl group can be single ring molecules, or can be substituents of larger molecules, or substituents of the same molecule (for intramolecular condensations).
The term xe2x80x9cnucleophilexe2x80x9d is recognized in the art, and as used herein means a chemical moiety having a reactive pair of electrons. Examples of nucleophiles include uncharged compounds such as alcohols, thiols, selenols, and amines, and charged moieties such as alkoxides, thiolates, carbanions, and a variety of other anions.
The term xe2x80x9celectrophilexe2x80x9d is art-recognized and refers to chemical moieties which can accept a pair of electrons from a nucleophile as defined above.
The terms xe2x80x9celectrophilic atomxe2x80x9d, xe2x80x9celectrophilic centerxe2x80x9d and xe2x80x9creactive centerxe2x80x9d as used herein refer to the atom of the substrate aryl moiety which is attacked by, and forms a new bond to, the nucleophile. In most (but not all) cases, this will also be the aryl ring atom from which the leaving group departs.
The term xe2x80x9celectron-withdrawing groupxe2x80x9d is recognized in the art, and denotes the tendency of a substituent to attract valence electrons from neighboring atoms, i.e., the substituent is electronegative with respect to neighboring atoms. A quantification of the level of electron-withdrawing capability is given by the Hammett sigma ("sgr") constant. This well known constant is described in many references, for instance, J. March, Advanced Organic Chemistry, McGraw Hill Book Company, New York, (1977 edition) pp. 251-259. The Hammett constant values are generally negative for electron donating groups ("sgr"[P]=xe2x88x920.66 for NH2) and positive for electron withdrawing groups ("sgr"[P]=0.78 for a nitro group), "sgr"[P] indicating para substitution. Exemplary electron-withdrawing groups include nitro, ketone, aldehyde, sulfonyl, trifluoromethyl, xe2x80x94CN, chloride, and the like. Exemplary electron-donating groups include amino, methoxy, and the like.
The term xe2x80x9creaction productxe2x80x9d means a compound which results from the reaction of a nucleophilic moiety of an aryl group and an electrophilic center of a substrate aryl group. In general, the term xe2x80x9creaction productxe2x80x9d will be used herein to refer to a stable, isolable diaryl ether product or the like, and not to unstable intermediates or transition states.
The term xe2x80x9ccomplexxe2x80x9d as used herein and in the claims means a coordination compound formed by the union of one or more electron-rich and electron-poor molecules or atoms capable of independent existence with one or more electronically poor molecules or atoms, each of which is also capable of independent existence.
The term xe2x80x9ccatalytic amountxe2x80x9d is recognized in the art and means a substoichiometric amount of reagent relative to a reactant. As used herein, a catalytic amount means from 0.0001 to 90 mole percent reagent relative to a reactant, more preferably from 0.001 to 50 mole percent, still more preferably from 0.01 to 10 mole percent, and even more preferably from 0.1 to 5 mole percent reagent to reactant.
The term xe2x80x9calkylxe2x80x9d refers to the radical of saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. In preferred embodiments, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., C1-C30 for straight chain. C3-C30 for branched chain), and more preferably 20 or fewer. Likewise, preferred cycloalkyls have from 3-10 carbon atoms in their ring structure, and more preferably have 5, 6 or 7 carbons in the ring structure.
Moreover, the term xe2x80x9calkylxe2x80x9d (or xe2x80x9clower alkylxe2x80x9d) as used throughout the specification and claims is intended to include both xe2x80x9cunsubstituted alkylsxe2x80x9d and xe2x80x9csubstituted alkylsxe2x80x9d, the latter of which refers to alkyl moieties having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. Such substituents can include, for example, a halogen, a hydroxyl, a carbonyl (such as a carboxyl, an ester, a formyl, or a ketone), a thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), an alkoxyl, a phosphoryl, a phosphonate; a phosphinate, an amino, an amido, an amidine, an imine, a cyano, a nitro, an azido, a sulfhydryl, an alkylthio, a sulfate, a sulfonate, a sulfamoyl, a sulfonamido, a sulfonyl, a heterocyclyl, an aralkyl, or an aromatic or heteroaromatic moiety. It will be understood by those skilled in the art that the moieties substituted on the hydrocarbon chain can themselves be substituted, if appropriate. For instance, the substituents of a substituted alkyl may include substituted and unsubstituted forms of amino, azido, imino, amido, phosphoryl (including phosphonate and phosphinate), sulfonyl (including sulfate, sulfonamido, sulfamoyl and sulfonate), and silyl groups, as well as ethers, alkylthios, carbonyls (including ketones, aldehydes, carboxylates, and esters), xe2x80x94CF3, xe2x80x94CN and the like. Exemplary substituted alkyls are described below. Cycloalkyls can be further substituted with alkyls, alkenyls, alkoxys, alkylthios, aminoalkyls, carbonyl-substituted alkyls, xe2x80x94CF3, xe2x80x94CN, and the like.
The term xe2x80x9caralkylxe2x80x9d, as used herein, refers to an alkyl group substituted with an aryl group (e.g., an aromatic or heteroaromatic group).
The terms xe2x80x9calkenylxe2x80x9d and xe2x80x9calkynylxe2x80x9d refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but that contain at least one double or triple bond respectively.
Unless the number of carbons is otherwise specified, xe2x80x9clower alkylxe2x80x9d as used herein means an alkyl group, as defined above, but having from one to ten carbons, more preferably from one to six carbon atoms in its backbone structure. Likewise, xe2x80x9clower alkenylxe2x80x9d and xe2x80x9clower alkynylxe2x80x9d have similar chain lengths. Preferred alkyl groups are lower alkyls. In preferred embodiments, a substituent designated herein as alkyl is a lower alkyl.
The term xe2x80x9carylxe2x80x9d as used herein includes 5-, 6- and 7-membered single-ring aromatic groups that may include from zero to four heteroatoms, for example, benzene, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine, and the like. Those aryl groups having heteroatoms in the ring structure may also be referred to as xe2x80x9caryl heterocyclesxe2x80x9d or xe2x80x9cheteroaromaticsxe2x80x9d. The aromatic ring can be substituted at one or more ring positions with such substituents as described above, for example, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, xe2x80x94CF3, xe2x80x94CN, or the like. The term xe2x80x9carylxe2x80x9d also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings (the rings are xe2x80x9cfused ringsxe2x80x9d) wherein at least one of the rings is aromatic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocyclyls.
The abbreviations Me, Et, Ph, Tf, Nf, Ts, Ms, and dba represent methyl, ethyl, phenyl, trifluoromethanesulfonyl, nonafluorobutanesulfonyl, p-toluenesulfonyl, methanesulfonyl, and dibenzylideneacetone, respectively. A more comprehensive list of the abbreviations utilized by organic chemists of ordinary skill in the art appears in the first issue of each volume of the Journal of Organic Chemistry; this list is typically presented in a table entitled Standard List of Abbreviations. The abbreviations contained in said list, and all abbreviations utilized by organic chemists of ordinary skill in the art are hereby incorporated by reference.
The terms ortho, meta and para apply to 1,2-, 1,3- and 1,4-disubstituted benzenes, respectively. For example, the names 1,2-dimethylbenzene and ortho-dimethylbenzene are synonymous.
The terms xe2x80x9cheterocyclylxe2x80x9d or xe2x80x9cheterocyclic groupxe2x80x9d refer to 3- to 10-membered ring structures, more preferably 3- to 7-membered rings, whose ring structures include one to four heteroatoms. Heterocycles can also be polycycles. Heterocyclyl groups include, for example, thiophene, thianthrene, furan, pyran, isobenzofuran, chromene, xanthene, phenoxathiin, pyrrole, imidazole, pyrazole, isothiazole, isoxazole, pyridine, pyrazine, pyrimidine, pyridazine, indolizine, isoindole, indole, indazole, purine, quinolizine, isoquinoline, quinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, pteridine, carbazole, carboline, phenanthridine, acridine, pyrimidine, phenanthroline, phenazine, phenarsazine, phenothiazine, furazan, phenoxazine, pyrrolidine, oxolane, thiolane, oxazole, piperidine, piperazine, morpholine, lactones, lactams such as azetidinones and pyrrolidinones, sultams, sultones, and the like. The heterocyclic ring can be substituted at one or more positions with such substituents as described above, as for example, halogen, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, ketone, aldehyde, ester, a heterocyclyl, an aromatic or heteroaromatic moiety, xe2x80x94CF3, xe2x80x94CN, or the like.
The terms xe2x80x9cpolycyclylxe2x80x9d or xe2x80x9cpolycyclic groupxe2x80x9d refer to two or more rings (e.g., cycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocyclyls) in which two or more carbons are common to two adjoining rings, e.g., the rings are xe2x80x9cfused ringsxe2x80x9d. Rings that are joined through non-adjacent atoms are termed xe2x80x9cbridgedxe2x80x9d rings. Each of the rings of the polycycle can be substituted with such substituents as described above, as for example, halogen, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, ketone, aldehyde, ester, a heterocyclyl, an aromatic or heteroaromatic moiety, xe2x80x94CF3, xe2x80x94CN, or the like.
The term xe2x80x9ccarbocyclexe2x80x9d, as used herein, refers to an aromatic or non-aromatic ring in which each atom of the ring is carbon.
The term xe2x80x9cheteroatomxe2x80x9d as used herein means an atom of any element other than carbon or hydrogen. Preferred heteroatoms are nitrogen, oxygen, sulfur and phosphorous.
As used herein, the term xe2x80x9cnitroxe2x80x9d means xe2x80x94NO2; the term xe2x80x9chalogenxe2x80x9d designates xe2x80x94F, xe2x80x94Cl, xe2x80x94Br or xe2x80x94I; the term xe2x80x9csulfhydrylxe2x80x9d means xe2x80x94SH; the term xe2x80x9chydroxylxe2x80x9d means xe2x80x94OH; and the term xe2x80x9csulfonylxe2x80x9d means xe2x80x94SO2xe2x80x94.
The terms xe2x80x9caminexe2x80x9d and xe2x80x9caminoxe2x80x9d are art recognized and refer to both unsubstituted and substituted amines, e.g., a moiety that can be represented by the general formula: 
wherein R9, R10 and Rxe2x80x210 each independently represent a hydrogen, an alkyl, an alkenyl, xe2x80x94(CH2)mxe2x80x94R8, or R9 and R10 taken together with the N atom to which they are attached complete a heterocycle having from 4 to 8 atoms in the ring structure; R8 represents an aryl, a cycloalkyl, a cycloalkenyl, a heterocycle or a polycycle; and m is zero or an integer in the range of 1 to 8. In preferred embodiments, only one of R9 or R10 can be a carbonyl, e.g., R9, R10 and the nitrogen together do not form an imide. In even more preferred embodiments, R9 and R10 (and optionally Rxe2x80x210) each independently represent a hydrogen, an alkyl, an alkenyl, or xe2x80x94(CH2)mxe2x80x94R8. Thus, the term xe2x80x9calkylaminexe2x80x9d as used herein means an amine group, as defined above, having a substituted or unsubstituted alkyl attached thereto, i.e., at least one of R9 and R10 is an alkyl group.
The term xe2x80x9cacylaminoxe2x80x9d is art-recognized and refers to a moiety that can be represented by the general formula: 
wherein R9 is as defined above, and Rxe2x80x211 represents a hydrogen, an alkyl, an alkenyl or xe2x80x94(CH2)mxe2x80x94 R8, where m and R8 are as defined above.
The term xe2x80x9camidoxe2x80x9d is art recognized as an amino-substituted carbonyl and includes a moiety that can be represented by the general formula: 
wherein R9, R10 are as defined above. Preferred embodiments of the amide will not include imides which may be unstable.
The term xe2x80x9calkylthioxe2x80x9d refers to an alkyl group, as defined above, having a sulfur radical attached thereto. In preferred embodiments, the xe2x80x9calkylthioxe2x80x9d moiety is represented by one of xe2x80x94Sxe2x80x94 alkyl, xe2x80x94S-alkenyl, xe2x80x94S-alkynyl, and xe2x80x94Sxe2x80x94(CH2)mxe2x80x94R8, wherein m and R8 are defined above. Representative alkylthio groups include methylthio, ethyl thio, and the like.
The term xe2x80x9ccarbonylxe2x80x9d is art recognized and includes such moieties as can be represented by the general formula: 
wherein X is a bond or represents an oxygen or a sulfur, and R11 represents a hydrogen, an alkyl, an alkenyl, xe2x80x94(CH2)mxe2x80x94R8 or a pharmaceutically acceptable salt, Rxe2x80x211 represents a hydrogen, an alkyl, an alkenyl or xe2x80x94(CH2)mxe2x80x94R8, where m and R8 are as defined above. Where X is an oxygen and R11 or Rxe2x80x211 is not hydrogen, the formula represents an xe2x80x9cesterxe2x80x9d. Where X is an oxygen, and R11 is as defined above, the moiety is referred to herein as a carboxyl group, and particularly when R11 is a hydrogen, the formula represents a xe2x80x9ccarboxylic acidxe2x80x9d. Where X is an oxygen, and Rxe2x80x211 is hydrogen, the formula represents a xe2x80x9cformatexe2x80x9d. In general, where the oxygen atom of the above formula is replaced by sulfur, the formula represents a xe2x80x9cthiolcarbonylxe2x80x9d group. Where X is a sulfur and R11 or Rxe2x80x211 is not hydrogen, the formula represents a xe2x80x9cthiolester.xe2x80x9d Where X is a sulfur and R11 is hydrogen, the formula represents a xe2x80x9cthiolcarboxylic acid.xe2x80x9d Where X is a sulfur and R11xe2x80x2 is hydrogen, the formula represents a xe2x80x9cthiolformate.xe2x80x9d On the other hand, where X is a bond, and R11 is not hydrogen, the above formula represents a xe2x80x9cketonexe2x80x9d group. Where X is a bond, and R11 is hydrogen, the above formula represents an xe2x80x9caldehydexe2x80x9d group.
The terms xe2x80x9calkoxylxe2x80x9d or xe2x80x9calkoxyxe2x80x9d as used herein refers to an alkyl group, as defined above, having an oxygen radical attached thereto. Representative alkoxyl groups include methoxy, ethoxy, propyloxy, tert-butoxy and the like. An xe2x80x9cetherxe2x80x9d is two hydrocarbons covalently linked by an oxygen. Accordingly, the substituent of an alkyl that renders that alkyl an ether is or resembles an alkoxyl, such as can be represented by one of xe2x80x94O-alkyl, xe2x80x94O-alkenyl, xe2x80x94O-alkynyl, xe2x80x94Oxe2x80x94(CH2)mxe2x80x94R8, where m and R8 are described above.
The term xe2x80x9csulfonatexe2x80x9d is art recognized and includes a moiety that can be represented by the general formula: 
in which R41 is an electron pair, hydrogen, alkyl, cycloalkyl, or aryl.
The terms triflyl, tosyl, mesyl, and nonaflyl are art-recognized and refer to trifluoromethanesulfonyl, p-toluenesulfonyl, methanesulfonyl, and nonafluorobutanesulfonyl groups, respectively. The terms triflate, tosylate, mesylate, and nonaflate are art-recognized and refer to trifluoromethanesulfonate ester, p-toluenesulfonate ester, methanesulfonate ester, and nonafluorobutanesulfonate ester functional groups and molecules that contain said groups, respectively.
The term xe2x80x9csulfatexe2x80x9d is art recognized and includes a moiety that can be represented by the general formula: 
in which R41 is as defined above.
The term xe2x80x9csulfonamidoxe2x80x9d is art recognized and includes a moiety that can be represented by the general formula: 
in which R9 and Rxe2x80x211 are as defined above.
The term xe2x80x9csulfamoylxe2x80x9d is art-recognized and includes a moiety that can be represented by the general formula: 
in which R9 and R10 are as defined above.
The terms xe2x80x9csulfoxidoxe2x80x9d or xe2x80x9csulfinylxe2x80x9d, as used herein, refers to a moiety that can be represented by the general formula: 
in which R44 is selected from the group consisting of hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, aralkyl, or aryl.
A xe2x80x9cphosphorylxe2x80x9d can in general be represented by the formula: 
wherein Q1 represented S or O, and R46 represents hydrogen, a lower alkyl or an aryl. When used to substitute, e.g., an alkyl, the phosphoryl group of the phosphorylalkyl can be represented by the general formula: 
wherein Q1 represented S or O, and each R46 independently represents hydrogen, a lower alkyl or an aryl, Q2 represents O, S or N. When Q1 is an S, the phosphoryl moiety is a xe2x80x9cphosphorothioatexe2x80x9d.
A xe2x80x9cphosphoramiditexe2x80x9d can be represented in the general formula: 
wherein R9 and R10 are as defined above, and Q2 represents O, S or N.
A xe2x80x9cphosphonamiditexe2x80x9d can be represented in the general formula: 
wherein R9 and R10 are as defined above, Q2 represents O, S or N, and R48 represents a lower alkyl or an aryl, Q2 represents O, S or N.
A xe2x80x9cselenoalkylxe2x80x9d refers to an alkyl group having a substituted seleno group attached thereto. Exemplary xe2x80x9cselenoethersxe2x80x9d which may be substituted on the alkyl are selected from one of xe2x80x94Se-alkyl, xe2x80x94Se-alkenyl, xe2x80x94Se-alkynyl, and xe2x80x94Sexe2x80x94(CH2)mxe2x80x94R8, m and R8 being defined above.
Analogous substitutions can be made to alkenyl and alkynyl groups to produce, for example, aminoalkenyls, aminoalkynyls, amidoalkenyls, amidoalkynyls, iminoalkenyls, iminoalkynyls, thioalkenyls, thioalkynyls, carbonyl-substituted alkenyls or alkynyls.
The phrase xe2x80x9cprotecting groupxe2x80x9d as used herein means temporary modifications of a potentially reactive functional group which protect it from undesired chemical transformations. Examples of such protecting groups include esters of carboxylic acids, silyl ethers of alcohols, and acetals and ketals of aldehydes and ketones, respectively. The field of protecting group chemistry has been reviewed (Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 2nd ed.; Wiley: New York, 1991).
It will be understood that xe2x80x9csubstitutionxe2x80x9d or xe2x80x9csubstituted withxe2x80x9d includes the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc.
As used herein, the term xe2x80x9csubstitutedxe2x80x9d is contemplated to include all permissible substituents of organic compounds. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, for example, those described hereinabove. The permissible substituents can be one or more and the same or different for appropriate organic compounds. For purposes of this invention, the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valencies of the heteroatoms. This invention is not intended to be limited in any manner by the permissible substituents of organic compounds.
A xe2x80x9cpolar solventxe2x80x9d means a solvent which has a dielectric constant (xcex5) of 2.9 or greater, such as DMF, THF, ethylene glycol dimethyl ether (DME), DMSO, acetone, acetonitrile, methanol, ethanol, isopropanol, n-propanol, t-butanol or 2-methoxyethyl ether. Preferred solvents are DMF, DME, NMP, and acetonitrile.
A xe2x80x9cpolar, aprotic solventxe2x80x9d means a polar solvent as defined above which has no available hydrogens to exchange with the compounds of this invention during reaction, for example DMF, acetonitrile, diglyme, DMSO, or THF.
An xe2x80x9caprotic solventxe2x80x9d means a non-nucleophilic solvent having a boiling point range above ambient temperature, preferably from about 25xc2x0 C. to about 190xc2x0 C., more preferably from about 80xc2x0 C. to about 160xc2x0 C., most preferably from about 80xc2x0 C. to 150xc2x0 C., at atmospheric pressure. Examples of such solvents are acetonitrile, toluene, DMF, diglyme, THF or DMSO.
For purposes of this invention, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 67th Ed., 1986-87, inside cover. Also for purposes of this invention, the term xe2x80x9chydrocarbonxe2x80x9d is contemplated to include all permissible compounds having at least one hydrogen and one carbon atom. In a broad aspect, the permissible hydrocarbons include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic organic compounds which can be substituted or unsubstituted.
Exemplary Catalyzed Reactions
As described above, one invention of the Applicants"" features a general metal-catalyzed cross-coupling reaction which comprises combining an aryl group (a xe2x80x9creactive arylxe2x80x9d) having a nucleophilic moiety, an aryl group (a xe2x80x9csubstrate arylxe2x80x9d) having an electrophilic center susceptible to attack by the nucleophilic moiety of the reactive aryl, at least a catalytic amount of a transition metal catalyst of particular characteristics (as described below), and an alkali salt derived from certain alkali metal or alkaline earth metals (the so-called xe2x80x9csoft ionsxe2x80x9d). The combination is maintained under conditions appropriate for the metal catalyst to catalyze the nucleophilic addition of the reactive aryl to the electrophilic atom of the substrate aryl.
Another related invention of the Applicants"" features a modified Ullmann reaction, similar to that described above, wherein the reaction includes the use of an acid activator, which as described herein, is an acid selected to help solubilize key intermediates in the condensation reaction. The use of such additives extends the applicability of the Ullman reaction to less soluble phenols and phenols containing electron-withdrawing groups. As described in the Examples, this modification can be combined with the modified Ullmann reaction above, e.g., with reaction profiles which include the use of a soft ion form of an alkali metal or alkaline earth metal salt, but should also be understood as useful with a wide range of other bases.
Each of the improvements to the Ullmann reaction, whether used together or separately, can be applied to cross-coupling reactions involving aryl halides and phenols. They may also be adapted to a wide range of other substituted aryls. Examples of the subject modified Ullmann reactions which may be carried out according to the present invention follow.
In an exemplary embodiment, the subject method can be used to synthesize diaryl ethers in an intermolecular reaction involving two substituted phenyl groups. For instance, the modified Ullmann reaction can be used to generate diaryl ether intermediates in the synthesis of antimitotic combretastatin analogs (see, for example, Boger et al. (1991) J Org Chem 56:4201). Compounds of this class have been shown to have antitumor activity. As illustrated below, 3-(3-hydroxy-4-alkoxyphenyl)-1-methoxy-1-propanol and 4-iodobenzaldehyde are reacted under appropriate conditions for the subject method. 
In another embodiment, the subject method can be used to synthesize diaryl ethers in which one or both of the aryl groups are polycyclic aromatics or heteroaromatic rings. As illustrated below, substituted forms of 3-bromo-2-phenylbenzo[b]thiophene and phenol can be reacted under the conditions of the subject Ullmann reactions to yield 3-phenoxy-2-phenylbenzo[b]thiophene derivatives. Such benzothiophene derivatives can be used in the treatment of post-menopausal syndrome, uterine fibroid disease, endometriosis and the like. See, for example, U.S. Pat. No. 5,510,498. 
In addition to intermolecular reactions, those skilled in the art will readily appreciate that the subject modified Ullmann reaction can be used in macrocyclization reactions, e.g., in an intramolecular reaction, in which the substrate and reactive aryl groups are part of the same molecule. Returning to the combrestatin synthesis, another means for generating combrestatin analogs is based on the implementation of an intramolecular Ullmann reaction for forming the cyclic 15-membered biaryl ether. As shown below, a (Z)-3-(4-iodophenyl)-2-propenyl 3-(3-hydroxyphenyl)propanoate can be reacted according to the criteria of the subject reaction to yield combrestatin D-2 analogs. 
As set out above, the subject Ullmann reaction can be carried out using reactive aryl groups having other nucleophilic groups in place of a reactive hydroxyl. In certain embodiments, the reactive aryl is an amine-substituted aryl. Aminopyridines are important in various fields of chemistry. They have been used as acyl transfer reagents in organic chemistry, and ligands in organic and organometallic chemistry. Additionally, aminopyridine derivatives have been used as fluorescent dyes and are biologically important as central nervous system stimulants. In an exemplary embodiment of the subject method, 3-bromoquinoline and N-methylaniline are reacted under the conditions of the subject method to form an N-methyl-N-phenyl-3-quinolinamine (reaction scheme 6): 
Another example of a reaction amenable to the conditions of the subject reaction is the synthesis of 1-cyano-3H-dibenz[f,ij] isoquinoline-2,7-diones. As shown below, the 1-cyano-6-bromyl-3H-dibenz[f,ij] isoquinoline-2,7-dione can be reacted with an aniline by the modified Ullmann reaction to form a diaryl amine, in this case a dye useful in toners for polyesters. 
In the exemplary transformation below, the subject modified Ullmann reaction is used in the synthesis of diaryl selenoethers and diaryl thioethers. For instance, the subject method can be used to generate the 6-pyridyl substituted pyrimidines of U.S. Pat. No. 5,278,167. Such compounds are useful in the treatment of retroviral infections. In the illustrative embodiment, 6-bromo-5-ethyl-1-(phenoxymethyl)-uracil and 3-pyridineselenol can be reacted according to the conditions of the modified Ullmann reaction to yield a 5-ethyl-1-(phenoxymethyl)-6-(3-pyridylselanyl)-uracil. 
Likewise, 6-bromo-5-ethyl-1-(phenoxymethyl)-uracil and 3-pyridinethiol can be reacted according to the present method in order to yield a 5-ethyl-1-(phenoxymethyl)-6-(3-pyridylsulfanyl)-uracil.
The modified Ullmann reaction of the present invention can also be used in the synthesis of polyether compounds. As illustrated below, a halogenated 3-oxapentane compound can be reacted with a dimethyldiphenylamine under the modified Ullmann reaction conditions of the present invention to produce a diphenylamine-capped polyether polymer. Such polymers can be used as polyconductive layers in electrophotographic photoconductors. See U.S. Pat. No. 5,158,850. 
As is clear from the above discussion, the products which may be produced by the modified Ullmann reaction of this invention can undergo further reaction(s) to afford desired derivatives thereof. Such permissible derivatization reactions can be carried out in accordance with conventional procedures known in the art. For example, potential derivatization reactions include esterification, oxidation of alcohols to aldehydes and acids, N-alkylation of amides, nitrile reduction, acylation of ketones by esters, acylation of amines and the like. To further illustrate, exemplary classes of pharmaceuticals which can be synthesized by a scheme including the subject reaction are cardiovascular drugs, nonsteroidal antiinflammatory drugs, central nervous system agents, and antihistaminics.
Exemplary Aryl Reactants
As can be seen from inspection of the illustrative reaction schemes above and in the appended examples, the subject methods are generally applicable for cross-coupling of a wide range of substituted aryl groups. Examples of aryl groups which can be used in the subject reactions, e.g., as either a substrate aryl group or a reactive aryl group, include but are not limited to, substituted forms of benzene, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, isothiazole, triazole, oxidiazole, pyrazole, pyridine, pyrazine, pyridazine, pyrimidine, triazine, benzofuran, isobenzofuran, indole, pyrano[3,4-b]pyrrole, indoxazine, phthalazine, anthranil, naphthalene, coumarin, isocoumarin (and other benzopyrones), quinoline, isoquinoline, cinnoline, quinazoline, naphthyridine, pyrido[3,4-b]pyridine, pyrido[3,2-b]pyridine, pyrido[4,3-b]pyridine, anthracene, benz[a]anthracene, acridine, azukene, heptalene, indacene, phenanthrene, chrysene, and the like.
In the case of the substrate aryl group (ArX), the aryl group is substituted with a leaving group which can be displaced by the nucleophilic group of the reactive aryl. In general, the leaving group is an electron-withdrawing group such as a halide, a cyano, a nitro, +N(R)3, diazonium moieties, or sulfonates (such as p-toluenesulfonates (tosylates), p-bromobenzenesulfonates (brosylates), p-nitrobenzenesulfonates (nosylates), methanesulfonates (mesylates), ammonioalkane sulfonate esters (betylates), alkyl fluorosulfonates, trifluoromethanesulfonates (triflates), nonafluorobutanesulfonates (nonaflates), 2,2,2-trifluoroethanesulfonates (tresylates), and the like). In preferred embodiments, the leaving group is a halide selected from iodine and bromine. Chlorine and fluorine can also be used as leaving groups, though other electronegative substitution on the aryl group may be required to activate those halogens as leaving groups in the subject metal cross-coupling reactions.
With respect to the reactive aryl group (Arxe2x80x2YH), the aryl group is substituted with a nucleophilic group, or a group that can be activated as a nucleophile, that can displace the leaving group of the substrate aryl in the subject reaction. Exemplary nucleophilic groups which can be used to substitute the reactive aryl include alcohols, thiols, selenyls, and amines. As an example of a group which can be activated as nucleophile, it is noted that in certain embodiments wherein a carboxylate is the nucleophile it will preferably masked as an ester or amide which can be activated under conditions consistent with its use in the subject modified Ullmann reaction.
In addition to the above activating substituents, both the substrate and reactive aryl groups may also be substituted with any number of a variety of groups. In this regard, the subject reaction can be represented in by the following general scheme. 
wherein, as described above, Ar and Arxe2x80x2 are aryl groups (of the same or different molecules), X is a leaving group, YH is nucleophilic group, and R1 and R2 represent one or more substitutions to the aryl groups Ar and Arxe2x80x2, respectively. Each incidence of substitution independently can be, as valence and stability permit, a halogen, a lower alkyl, a lower alkenyl, a lower alkynyl, a carbonyl (e.g., an ester, a carboxylate, or a formate), a thiocarbonyl (e.g., a thiolester, a thiolcarboxylate, or a thiolformate), a ketyl, an aldehyde, an amino, an acylamino, an amido, an amidino, a cyano, a nitro, an azido, a sulfonyl, a sulfoxido, a sulfate, a sulfonate, a sulfamoyl, a sulfonamido, a phosphoryl, a phosphonate, a phosphinate, xe2x80x94(CH2)mxe2x80x94R8, xe2x80x94(CH2)mxe2x80x94OH, xe2x80x94(CH2)mxe2x80x94 O-lower alkyl, xe2x80x94(CH2)mxe2x80x94O-lower alkenyl, xe2x80x94(CH2)mxe2x80x94Oxe2x80x94(CH2)nxe2x80x94R8, xe2x80x94(CH2)mxe2x80x94SH, xe2x80x94(CH2)mxe2x80x94S-lower alkyl, xe2x80x94(CH2)mxe2x80x94S-lower alkenyl, xe2x80x94(CH2)mxe2x80x94Sxe2x80x94(CH2)nxe2x80x94R8, or protecting groups of the above or a solid or polymeric support; R8 represents a substituted or unsubstituted aryl, aralkyl, cycloalkyl, cycloalkenyl, or heterocycle; and n and m are independently for each occurrence zero or an integer in the range of 1 to 6.
Metal-containing Catalysts
The catalysts employed in the subject method involve the use of metals which can mediate cross-coupling of the aryl groups ArX and Arxe2x80x2YH as defined above. In general, catalysts intended by the present invention can be characterized in terms of a number of features. For instance, in preferred embodiments the metal should be capable of forming an adduct with the nucleophilic group of the reactive aryl in order to activate it for attack at an electrophilic center of the substrate aryl. For instance, a possible mechanism for the present reaction provides the metal catalyst in an intermediate form of an aryloxy adduct Arxe2x80x2OM (where M is the metal).
In general, any transition metal (e.g., having d electrons) may be used to form the catalyst, e.g., a metal selected from one of Groups 3-12 of the periodic table or from the lanthanide series. However, in preferred embodiments, the metal will be selected from the group of late transition metals, e.g. preferably from Groups 5-12 and even more preferably Groups 7-11. For example, suitable metals include Cu, Co, Mn, Fe, and Ni. The particular form of the metal to be used in the reaction is selected to provide, under the reaction conditions, a metal center which is coordinately unsaturated and not in its highest oxidation state. Particularly preferred metals are copper, especially forms of the catalyst providing Cu(I) cations or which can be reduced or oxidized (as appropriate) to Cu(I) under the reaction conditions. For instance, Cu(I) can be produced by reaction between Cu(O) and Cu(II). For example, Jenkins et al. (1974) J. Amer. Chem. Soc., 94:843 reports the preparation of cuprous triflate by the reaction of cupric triflate with copper metal in deoxygenated acetonitrile/triflic acid solution.
Without being bound by any particular theory, it is believed that the active copper species is Cu(I). However, by virtue of the oxidation-reduction processes occurring in the reaction mixture, the metal catalyst can be provided as its metallic form, as a metal salt, or as a metal oxide. However, the role of such factors as the solubility of the copper compound in the solvent system play a role in optimizing the selection of a copper catalyst.
In the case of metal salts, the counterion(s) can be selected from a variety of anion(s) of organic or inorganic origin, that can be, though not necessarily, weakly-nucleophilic (e.g., a weak base whose conjugate acid has a pKa less than 1). Weakly or non-nucleophilic stabilizing ions are preferred to avoid complicating side reactions of the counter ion involving attack at or addition to the electrophilic center of the substrate aryl.
Examples of suitable metal salts include those derived from arylsulfonic acids, alkylsulfonic acids, halogenated aryl- and alkylsulfonic acids, tetrafluoroboric acid, sulfuric acid, heteropolyacids, and halosulfonic acids. Sulfonic acid salts, especially triflate salts, are particularly preferred. For instance, suitable anions include fluorosulfate (FSO3xe2x88x92); alkyl sulfates, especially methyl sulfate (CH3SO3xe2x88x92); perfluoroalkylsulfonates, such as triflate (CF3SO3xe2x88x92) and nonaflate (C4F9SO3xe2x88x92); arenesulfonates, especially tosylate (i.e., p-toluenesulfonate; CH3C6H4SO3xe2x88x92); alkylcarboxylates: perfluoroalkylcarboxylates; tetrafluoroborate (BF4xe2x88x92); tetraphenylborate (Ph4Bxe2x88x92); hexafluorophosphate (PF6xe2x88x92); hexafluoroantimonate (SbF6xe2x88x92); chlorate (ClO3xe2x88x92); and sulfate (SO4xe2x88x922).
To further illustrate, suitable inorganic copper ion sources include copper (II) triflate, copper tetrafluoroborate, copper p-toluenesulfonate, copper chloride, and the dihydrate thereof; copper fluoride, and the dihydrate thereof; copper fluorosilicate, and the hexahydrate thereof; copper sulphate, and the pentahydrate thereof; copper nitrate and the tri- and hexa-hydrates thereof; and also less popular copper salts, such as copper bromide; copper metaborate; copper bromate; copper chlorate; copper iodate; copper fluorophosphate, or mixtures thereof.
Preferred copper salts of organic acids include copper acetate, copper formate, copper benzoate, copper citrate, copper tartrate, copper lactate, copper malate, copper lysinate, copper mandelate, copper sorbate, copper pantothenate, copper gluconate, copper phytate, copper glycerophosphate, copper cinnamate, copper butyrate, copper propionate, copper laurate, copper oxalate, copper salicylate, copper glycinate, copper bis-glycinate or mixtures thereof. Mixtures of inorganic or organic salts may also be used.
It is understood, that the listed salts are only a small portion of the metal salts usable in the process. For the production of the catalysts of the invention, cost-effective inorganic salts are preferred.
Metallic copper can also be used, and can be obtained for use in the present method by, e.g., reducing CuSO4 with zinc or by electrolytic deposition.
The preferred amount of catalyst to be used depends on many factors, including the catalyst type, reaction solvent, desired reaction rate, the type of substrate and reactive aryls used, reaction temperature, and other factors. Generally, it is preferred to use an amount of metal salt within the range of about 0.01% to 10 mol %.
Co-catalysts
In addition to the transition metal catalysts, the subject reaction preferably also includes a Lewis basic co-catalyst. While not wishing to be bound by any particular theory, experimental data supports the notion that the co-catalyst can be selected to be an agent having a functional group including an electron pair donor (Lewis base) capable of coordination with the transition metal. The co-catalyst may control the stability and electron transfer properties of the transition metal catalyst, and/or stabilize the metal intermediates.
Lewis basic moieties useful as a co-catalyst in this invention include linear and cyclic compounds which include at least one Lewis basic group. Such functionalities can be provided, in general, by such groups as ethers, esters, nitrites, ketones, amides, amines, phosphorus compounds and alcohols of up to 30 carbon atoms, and other organic Lewis bases which are soluble or can be suspended in the reaction mixture or solvent. Examples are alkyl and aryl acetates, alkyl acrylates, alkyl methacrylates, alkyl undecylenates, acetonitrile, benzonitrile, acrylonitrile, acetylacetone, tetrahydrofuran, pyridine, N,N-dimethylformamide, thiophene, ethyl ether, propyl ethers, diphenyl ether, triethylamine, phenylacetylene, organic phosphorus compounds of from 1 to 30 carbon atoms and monohydroxylic and dihydroxylicalcohols of from 1 to 30 carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, n-pentyl alcohol, isopentyl alcohol, 2-methyl-1-butyl alcohol, 2-methyl-2-butyl alcohol, n-hexyl alcohol, n-heptyl alcohol, n-octyl alcohol, isooctyl alcohol, 2-ethylhexyl alcohol, n-nonyl alcohol, n-decyl alcohol, 1,5-aentanediol, 1,6-hexanediol, allyl alcohol, crotyl alcohol, 3-hexene-1-ol, citronellol, cyclopentanol, cyclohexanol, salicyl alcohol, benzyl alcohol, phenethyl alcohol, cinnamyl alcohol, and the like, and ketones, such as acetone, methyl ethyl ketone, diisopropyl ketone, benzophenone, acetophenone, dibenzyl ketone and the like.
The Lewis base is preferably an ester, such as an alkyl acetate, instead of an alcohol. A nitrile such as acetonitrile is preferable to pyridine.
As with the transition metal catalyst, the Lewis basic co-catalyst can be provided in the reaction mixture at sub-stoichiometric amounts. The preferred amount of co-catalyst to be used depends on many factors, including the catalyst type, reaction solvent, desired reaction rate, the type of substrate and reactive aryls used, reaction temperature, and other factors. Generally, it is preferred to use an amount of the Lewis base within the range of about 0.01% to 10 mol %.
Alkali or Alkaline Earth Salt
In one embodiment, the subject method is carried out in the presence of an inorganic salt selected from alkali metal salts and alkaline earth metal salts, in preferred embodiments the metal cation is characterized by having electrons in the 4th or higher quantum level. The alkali salt, such as cesium carbonate, increases the solubility of the metal catalyst intermediates involving Arxe2x80x2YH. The cationic portion is a xe2x80x9csoft cationxe2x80x9d (e.g., of large ionic radius) from the alkali metal or alkaline earth series, which cation can form an intermediate complex with a metal catalyst-activated Arxe2x80x2YH. For instance, the cation can be an alkali metal such as Rb, Cs and Fr, or an alkaline earth metal such as Sr, Ba and Ra. In many cases, the addition of such an alkali salt to the reaction mixture contributes to eliminating the need to form a phenoxide anion or the like prior to the condensation reaction, a salient feature of the reaction with respect to running the reaction in non-polar solvents.
Under certain of the reaction conditions, the anionic portion of the salt will preferably be a strong enough base to activate Arxe2x80x2YH. For instance, where the reactive aryl is a phenol, the base portion of the salt should be strong enough to deprotonate the phenolic group. Therefore, for most reactions involving an arenol reactant, a base, the conjugate acid of which has a pKa of at least about 10, is required. For aryls groups such as anilines a stronger base may be needed, whereas for arylthiols a somewhat weaker base may be used.
In exemplary embodiments, the base can be: a carbonate of an alkali metal, e.g., cesium carbonate or cesium bicarbonate; an alkali or alkaline earth metal hydroxide; an alkali metal or alkaline earth metal alkoxides or amides, such as cesium methoxide, cesium ethoxide, or cesium tert-butoxide.
Acid Activator
Another aspect of the invention relates to the use of an acid activator, which as described above, is an acid selected to help solubilize key intermediates in the condensation reaction. In general, the activator will include an acidic group, e.g., with a pKa less than 5, capable of producing a conjugate base which, under the reaction conditions, is a strong enough Lewis base to donate an electron pair to the transition metal of the catalyst. Moreover, the activator will include substituents which generally render it, and its transition metal complexes, more soluble in the reaction solvent than the catalyst by itself. By use of such reaction additives, the resulting new procedure allows, for the first the time, an efficient Ullmann reaction with less reactive and/or less soluble phenols.
In general, the acid activator is a compound that includes an anionic electron pair donor (Lewis basic) group which, though not being bound by a particular theory, is a potential ligand for metal catalyst complexes. To further illustrate, for example, suitable acid activators of the subject method include certain monofunctional or polyfunctional carboxylic acids, amides, hydroxamic acids, phosphoric acids, phosphonic acids, phosphinic acids, sulfonic acids, sulfinic acids, sulfenic acids, alkylboronic acids, arylboronic acids, silicic acids, alcohols, and thiols. In other embodiments, the acid activator includes a moiety which, under the reaction conditions, can be converted to a strongly acid group, such as an acid anhydride. In still other embodiments, the activator can be a phenol, e.g., a sterically crowded phenol. The electron donating compounds as described hereinabove may be used singly or in combination of two or more.
The acid activator can be used in the improved Ullmann reactions set out above, e.g., conjointly with an alkali salt such as cesium carbonate. Additionally, the acid activator can be used to improve the reaction for less reactive and/or less soluble phenols in the presence of a wide range of other bases, such as alkali salts generally.
In preferred embodiments, the acid activator is used in the subject reaction in the presence of molecular sieves such as zeolites. Zeolite molecular sieves are discussed in great detail in D. W. Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Company, Malabar, Fla. (1984).
Reaction Conditions
The modified Ullmann reactions of the present invention may be performed under a wide range of conditions, though it will be understood that the solvents and temperature ranges recited herein are not limitative and only correspond to a preferred mode of the process of the invention.
In general, it will be desirable that reactions are run using mild conditions which will not adversely affect the reactants, the catalyst, or the product. For example, the reaction temperature influences the speed of the reaction, as well as the stability of the reactants and catalyst. The reactions will usually be run at temperatures in the range of 25xc2x0 C. to 200xc2x0 C., more preferably in the range 75xc2x0 C. to 150xc2x0 C.
In general, the subject reactions are carried out in a liquid reaction medium. However, the reactions may be run without addition of solvent. Alternatively, the reactions may be run in an inert solvent, preferably one in which the reaction ingredients, including the catalyst, are substantially soluble. Suitable solvents include ethers such as diethyl ether, 1,2-dimethoxyethane, diglyme, t-butyl methyl ether, tetrahydrofuran and the like; halogenated solvents such as chloroform, dichloromethane, dichloroethane, chlorobenzene, and the like; aliphatic or aromatic hydrocarbon solvents such as benzene, xylene, toluene, hexane, pentane and the like; esters and ketones such as ethyl acetate, acetone, and 2-butanone; polar aprotic solvents such as acetonitrile, dimethylsulfoxide, dimethylformamide and the like; or combinations of two or more solvents.
The invention also contemplates reaction in a biphasic mixture of solvents, in an emulsion or suspension, or reaction in a lipid vesicle or bilayer. In certain embodiments, it may be preferred to perform the catalyzed reactions on a solid phase, e.g., with one of the reactants anchored to a solid support.
In certain embodiments it is preferable to perform the reactions under an inert atmosphere of a gas such as nitrogen or argon.
The reaction processes of the present invention can be conducted in continuous, semi-continuous or batch fashion and may involve a liquid recycle operation as desired. The processes of this invention are preferably conducted in batch fashion. Likewise, the manner or order of addition of the reaction ingredients, catalyst and solvent are also not generally critical and may be accomplished in any conventional fashion.
The reaction can be conducted in a single reaction zone or in a plurality of reaction zones, in series or in parallel or it may be conducted batchwise or continuously in an elongated tubular zone or series of such zones. The materials of construction employed should be inert to the starting materials during the reaction and the fabrication of the equipment should be able to withstand the reaction temperatures and pressures. Means to introduce and/or adjust the quantity of starting materials or ingredients introduced batchwise or continuously into the reaction zone during the course of the reaction can be conveniently utilized in the processes especially to maintain the desired molar ratio of the starting materials. The reaction steps may be effected by the incremental addition of one of the starting materials to the other. Also, the reaction steps can be combined by the joint addition of the starting materials to the metal catalyst. When complete conversion is not desired or not obtainable, the starting materials can be separated from the product and then recycled back into the reaction zone.
The processes may be conducted in either glass lined, stainless steel or similar type reaction equipment. The reaction zone may be fitted with one or more internal and/or external heat exchanger(s) in order to control undue temperature fluctuations, or to prevent any possible xe2x80x9crunawayxe2x80x9d reaction temperatures.
Furthermore, one or more of the reactants can be immobilized or incorporated into a polymer or other insoluble matrix by, for example, derivativation with one or more of substituents of the aryl group.
Combinatorial Libraries
The subject reactions readily lend themselves to the creation of combinatorial libraries of compounds for the screening of pharmaceutical, agrochemical or other biological or medically-related activity or material-related qualities. A combinatorial library for the purposes of the present invention is a mixture of chemically related compounds which may be screened together for a desired property; said libraries may be in solution or covalently linked to a solid support. The preparation of many related compounds in a single reaction greatly reduces and simplifies the number of screening processes which need to be carried out. Screening for the appropriate biological, pharmaceutical, agrochemical or physical property may be done by conventional methods.
Diversity in a library can be created at a variety of different levels. For instance, the substrate aryl groups used in a combinatorial approach can be diverse in terms of the core aryl moiety, e.g., a variegation in terms of the ring structure, and/or can be varied with respect to the other substituents.
A variety of techniques are available in the art for generating combinatorial libraries of small organic molecules. See, for example, Blondelle et al. (1995) Trends Anal. Chem. 14:83; the Affymax U.S. Pat. Nos. 5,359,115 and 5,362,899: the Ellman U.S. Pat. No. 5,288,514: the Still et al. PCT publication WO 94/08051; Chen et al. (1994) JACS 116:2661: Kerr et al. (1993) JACS 115:252; PCT publications WO92/10092, WO93/09668 and WO091/07087; and the Lerner et al. PCT publication WO93/20242). Accordingly, a variety of libraries on the order of about 16 to 1,000,000 or more diversomers can be synthesized and screened for a particular activity or property.
In an exemplary embodiment, a library of substituted diversomers can be synthesized using the subject reactions adapted to the techniques described in the Still et al. PCT publication WO 94/08051, e.g., being linked to a polymer bead by a hydrolyzable or photolyzable group, e.g., located at one of the positions of substrate. According to the Still et al. technique, the library is synthesized on a set of beads, each bead including a set of tags identifying the particular diversomer on that bead. In one embodiment, which is particularly suitable for discovering enzyme inhibitors, the beads can be dispersed on the surface of a permeable membrane, and the diversomers released from the beads by lysis of the bead linker. The diversomer from each bead will diffuse across the membrane to an assay zone, where it will interact with an enzyme assay. Detailed descriptions of a number of combinatorial methodologies are provided below.
Direct Characterization
A growing trend in the field of combinatorial chemistry is to exploit the sensitivity of techniques such as mass spectrometry (MS), e.g., which can be used to characterize sub-femtomolar amounts of a compound, and to directly determine the chemical constitution of a compound selected from a combinatorial library. For instance, where the library is provided on an insoluble support matrix, discrete populations of compounds can be first released from the support and characterized by MS. In other embodiments, as part of the MS sample preparation technique, such MS techniques as MALDI can be used to release a compound from the matrix, particularly where a labile bond is used originally to tether the compound to the matrix. For instance, a bead selected from a library can be irradiated in a MALDI step in order to release the diversomer from the matrix, and ionize the diversomer for MS analysis.
B) Multipin Synthesis
The libraries of the subject method can take the multipin library format. Briefly, Geysen and co-workers (Geysen et al. (1984) PNAS 81:3998-4002) introduced a method for generating compound libraries by a parallel synthesis on polyacrylic acid-grated polyethylene pins arrayed in the microtitre plate format. The Geysen technique can be used to synthesize and screen thousands of compounds per week using the multipin method, and the tethered compounds may be reused in many assays. Appropriate linker moieties can also been appended to the pins so that the compounds may be cleaved from the supports after synthesis for assessment of purity and further evaluation (c.f., Bray et al. (1990) Tetrahedron Lett 31:5811-5814; Valerio et al. (1991) Anal Biochem 197:168-177; Bray et al. (1991) Tetrahedron Lett 32:6163-6166).
C) Divide-Couple-Recombine
In yet another embodiment, a variegated library of compounds can be provided on a set of beads utilizing the strategy of divide-couple-recombine (see, e.g., Houghten (1985) PNAS 82:5131-5135; and U.S. Pat. Nos. 4,631,211; 5,440,016; 5,480,971). Briefly, as the name implies, at each synthesis step where degeneracy is introduced into the library, the beads are divided into separate groups equal to the number of different substituents to be added at a particular position in the library, the different substituents coupled in separate reactions, and the beads recombined into one pool for the next iteration.
In one embodiment, the divide-couple-recombine strategy can be carried out using an analogous approach to the so-called xe2x80x9ctea bagxe2x80x9d method first developed by Houghten, where compound synthesis occurs on resin sealed inside porous polypropylene bags (Houghten et al. (1986) PNAS 82:5131-5135). Substituents are coupled to the compound-bearing resins by placing the bags in appropriate reaction solutions, while all common steps such as resin washing and deprotection are performed simultaneously in one reaction vessel. At the end of the synthesis, each bag contains a single compound.
D) Combinatorial Libraries by Light-Directed, Spatially Addressable Parallel Chemical Synthesis
A scheme of combinatorial synthesis in which the identity of a compound is given by its locations on a synthesis substrate is termed a spatially-addressable synthesis. In one embodiment, the combinatorial process is carried out by controlling the addition of a chemical reagent to specific locations on a solid support (Dower et al. (1991) Annu Rep Med Chem 26:271-280; Fodor, S. P. A. (1991) Science 251:767; Pirrung et al. (1992) U.S. Pat. No. 5,143,854; Jacobs et al. (1994) Trends Biotechnol 12:19-26). The spatial resolution of photolithography affords miniaturization. This technique can be carried out through the use protection/deprotection reactions with photolabile protecting groups.
The key points of this technology are illustrated in Gallop et al. (1994) J Med Chem 37:1233-1251. A synthesis substrate is prepared for coupling through the covalent attachment of photolabile nitroveratryloxycarbonyl (NVOC) protected amino linkers or other photolabile linkers. Light is used to selectively activate a specified region of the synthesis support for coupling. Removal of the photolabile protecting groups by light (deprotection) results in activation of selected areas. After activation, the first of a set of amino acid analogs, each bearing a photolabile protecting group on the amino terminus, is exposed to the entire surface. Coupling only occurs in regions that were addressed by light in the preceding step. The reaction is stopped, the plates washed, and the substrate is again illuminated through a second mask, activating a different region for reaction with a second protected building block. The pattern of masks and the sequence of reactants define the products and their locations. Since this process utilizes photolithography techniques, the number of compounds that can be synthesized is limited only by the number of synthesis sites that can be addressed with appropriate resolution. The position of each compound is precisely known; hence, its interactions with other molecules can be directly assessed.
In a light-directed chemical synthesis, the products depend on the pattern of illumination and on the order of addition of reactants. By varying the lithographic patterns, many different sets of test compounds can be synthesized simultaneously; this characteristic leads to the generation of many different masking strategies.
E) Encoded Combinatorial Libraries
In yet another embodiment, the subject method utilizes a compound library provided with an encoded tagging system. A recent improvement in the identification of active compounds from combinatorial libraries employs chemical indexing systems using tags that uniquely encode the reaction steps a given bead has undergone and, by inference, the structure it carries. Conceptually, this approach mimics phage display libraries, where activity derives from expressed peptides, but the structures of the active peptides are deduced from the corresponding genomic DNA sequence. The first encoding of synthetic combinatorial libraries employed DNA as the code. A variety of other forms of encoding have been reported, including encoding with sequenceable bio-oligomers (e.g., oligonucleotides and peptides), and binary encoding with additional non-sequenceable tags.
1) Tagging with sequenceable bio-oligomers
The principle of using oligonucleotides to encode combinatorial synthetic libraries was described in 1992 (Brenner et al. (1992) PNAS 89:5381-5383), and an example of such a library appeared the following year (Needles et al. (1993) PNAS 90:10700-10704). A combinatorial library of nominally 77 (=823,543) peptides composed of all combinations of Arg, Gln, Phe, Lys, Val, D-Val and Thr (three-letter amino acid code), each of which was encoded by a specific dinucleotide (TA, TC, CT, AT, TT, CA and AC, respectively), was prepared by a series of alternating rounds of peptide and oligonucleotide synthesis on solid support. In this work, the amine linking functionality on the bead was specifically differentiated toward peptide or oligonucleotide synthesis by simultaneously preincubating the beads with reagents that generate protected OH groups for oligonucleotide synthesis and protected NH2 groups for peptide synthesis (here, in a ratio of 1:20). When complete, the tags each consisted of 69-mers, 14 units of which carried the code. The bead-bound library was incubated with a fluorescently labeled antibody, and beads containing bound antibody that fluoresced strongly were harvested by fluorescence-activated cell sorting (FACS). The DNA tags were amplified by PCR and sequenced, and the predicted peptides were synthesized. Following such techniques, compound libraries can be derived for use in the subject method, where the oligonucleotide sequence of the tag identifies the sequential combinatorial reactions that a particular bead underwent, and therefore provides the identity of the compound on the bead.
The use of oligonucleotide tags permits exquisitely sensitive tag analysis. Even so, the method requires careful choice of orthogonal sets of protecting groups required for alternating co-synthesis of the tag and the library member. Furthermore, the chemical lability of the tag, particularly the phosphate and sugar anomeric linkages, may limit the choice of reagents and conditions that can be employed for the synthesis of non-oligomeric libraries. In preferred embodiments, the libraries employ linkers permitting selective detachment of the test compound library member for assay.
Peptides have also been employed as tagging molecules for combinatorial libraries. Two exemplary approaches are described in the art, both of which employ branched linkers to solid phase upon which coding and ligand strands are alternately elaborated. In the first approach (Kerr JM et al. (1993) J Am Chem Soc 115 :2529-2531), orthogonality in synthesis is achieved by employing acid-labile protection for the coding strand and base-labile protection for the compound strand.
In an alternative approach (Nikolaiev et al. (1993) Pept Res 6:161-170), branched linkers are employed so that the coding unit and the test compound can both be attached to the same functional group on the resin. In one embodiment, a cleavable linker can be placed between the branch point and the bead so that cleavage releases a molecule containing both code and the compound (Ptek et al. (1991) Tetrahedron Lett 32:3891-3894). In another embodiment, the cleavable linker can be placed so that the test compound can be selectively separated from the bead, leaving the code behind. This last construct is particularly valuable because it permits screening of the test compound without potential interference of the coding groups. Examples in the art of independent cleavage and sequencing of peptide library members and their corresponding tags has confirmed that the tags can accurately predict the peptide structure.
2) Non-sequenceable Tagging: Binary Encoding
An alternative form of encoding the test compound library employs a set of non-sequencable electrophoric tagging molecules that are used as a binary code (Ohlmeyer et al. (1993) PNAS 90:10922-10926). Exemplary tags are haloaromatic alkyl ethers that are detectable as their trimethylsilyl ethers at less than femtomolar levels by electron capture gas chromatography (ECGC). Variations in the length of the alkyl chain, as well as the nature and position of the aromatic halide substituents, permit the synthesis of at least 40 such tags, which in principle can encode 240 (e.g., upwards of 1012) different molecules. In the original report (Ohlmeyer et al., supra) the tags were bound to about 1% of the available amine groups of a peptide library via a photocleavable o-nitrobenzyl linker. This approach is convenient when preparing combinatorial libraries of peptide-like or other amine-containing molecules. A more versatile system has, however, been developed that permits encoding of essentially any combinatorial library. Here, the compound would be attached to the solid support via the photocleavable linker and the tag is attached through a catechol ether linker via carbene insertion into the bead matrix (Nestler et al. (1994) J Org Chem 59:4723-4724). This orthogonal attachment strategy permits the selective detachment of library members for assay in solution and subsequent decoding by ECGC after oxidative detachment of the tag sets.
Although several amide-linked libraries in the art employ binary encoding with the electrophoric tags attached to amine groups, attaching these tags directly to the bead matrix provides far greater versatility in the structures that can be prepared in encoded combinatorial libraries. Attached in this way, the tags and their linker are nearly as unreactive as the bead matrix itself. Two binary-encoded combinatorial libraries have been reported where the electrophoric tags are attached directly to the solid phase (Ohlmeyer et al. (1995) PNAS 92:6027-6031) and provide guidance for generating the subject compound library. Both libraries were constructed using an orthogonal attachment strategy in which the library member was linked to the solid support by a photolabile linker and the tags were attached through a linker cleavable only by vigorous oxidation. Because the library members can be repetitively partially photoeluted from the solid support, library members can be utilized in multiple assays. Successive photoelution also permits a very high throughput iterative screening strategy: first, multiple beads are placed in 96-well microtiter plates; second, compounds are partially detached and transferred to assay plates; third, a metal binding assay identifies the active wells; fourth, the corresponding beads are rearrayed singly into new microtiter plates; fifth, single active compounds are identified; and sixth, the structures are decoded.